Electron ionization internal energy distribution

In electron ionization, internal energy distribution is very important This means that ions possess very different internal energies among the formed M+- ion population. This phenomenon occurs because (1) all the molecules M do not arrive in the source with the same energy because they clash and collide with the omnipresent helium atoms and residual atmospheric molecules, and (2) all the electrons emitted by the filament do not collide with the molecules with the same kinetic energy (70 eV is the average value). These electrons have different speed characteristics according to the part of the filament that emits them they are also subject to collisions with helium atoms and HjO, Nj, and O2 molecules present in the source. 

Data taken for two different internal energy distributions in CH4 + produced by ionization with 13.5- and 80-volt ionizing electrons, respectively. 

Fig. 2.4. Definition of appearance energy and visualization of changes in internal energy distributions, F(E), of relevant species upon electron ionization and subsequent fragmentation. The energy scale is shown compressed for the ions.

Also, a rigorous treatment of isotope effects within the framework of QET reveals that the assumption /muZ/mD = hZZ d represents a simplification. [69] It is only valid for when the species studied populate a small internal energy distribution, e.g., as metastable ions do, whereas wide internal energy distributions, e.g., those of ions fragmenting in the ion source after 70 eV electron ionization, may cause erroneous results. This is because the fc(E) functions of isotopic reactions are not truly parallel, [76] but they fulfill this requirement over a small range of internal energies (Figs. 2.17 and 2.18)... 

At first sight, it seems surprising to observe competitive reactions within the same complex. However, it must be noticed that in the ionization processes the internal energy distribution within the ions can be broad since the cluster s geometries in the Sj excited state and the ionic state can be very different. This can be seen by the ionization threshold measurements which do not exhibit clear onsets. Therefore, the presence of competitive processes can be explained by different barrier heights for the different channels. When the ions are prepared below one barrier and above the other one, only one product will be observed. Due to this broad internal energy distribution, on average, many channels can be detected. Coincidence detection of the zero kinetic electron and the product ions... 

Using an ordinary El mass spectrometer with electron energies in the range 50—100 eV, the internal energy distribution, P(E), of the molecular ion is typically extremely broad (possibly tens of eV) and poorly defined. Both direct ionization and autoionization contribute to formation of molecular ions [517]. 

Figure 3.2. Internal energy distribution of ions generated by electron ionization.

Chemical ionization generally supplies fewer ions than electron ionization, in terms of genre and quantity because the internal energy distribution is weaker than in El. Thus, the recorded ionic current in Cl is generally inferior to that recorded in El chromatograms. Nevertheless, Cl can sometimes supply a detection threshold inferior to that of the corresponding El method. This depends on two faetors. 

Fig. 2.2. Electron ionization can be represented by a vertical line in this diagram. Thus, ions are formed in a vibrationaUy excited state if the intemuclear distance of the excited state is longer than in the ground state. Ions having internal energies below the dissociation energy D remain stable, whereas fragmentation will occur above. In few cases, ions are unstable, i.e., there is no minimum on their potential energy curve. The lower part schematically shows the distribution of Franck-Condon factors, fyc, for various transitions.

In this method, the sample is thermally vaporized and approximately 10 Torr of its vapors enter the ion source volume where they are ionized by collision with an electron beam of (typically) 70 eV kinetic energy. Electron ionization can produce intact molecular radical cations, M% by ejection of an electron from the sample molecules (Eq. 1.1). This process has a yield of -0.01% and deposits a wide distribution of internal enei es to the newly formed molecular ions as a result, many M are formed excited enough to yield a number of fragment ions (Eq. 1.2) via competitive (F, F2, F3) and consecutive (f, fb, O decompositions. 

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